The present invention concerns a process and an apparatus for controlling the supply of alumina to a cell for the production of aluminum by electrolysis using the Hall-Heroult process.
Supply means are known for supplying alumina to an electrolytic cell for producing alumina. They are described in particular in French Pat. Nos. 1,245,598; 1,526,766; 2,036,896; 2,099,343; and 2,264,098 which correspond to U.S. Pat. Nos. 3,216,918; 3,372,106; 3,679,555; 3,681,229; and 3,901,787, respectively; and in additional U.S. Pat. Nos. 3,400,062 and 3,681,229.
The apparatuses which are normally used permit transportation of the alumina to one or more supply points per cell; the alumina is poured on to the crust which is formed at the surface of the electrolyte and it is introduced into the electrolyte by blows applied by a plunger which periodically moves down and breaks the crust or keeps the hole open, in which case the alumina is poured directly into the molten electrolyte.
In general, the downward movement of the plunger is timed. The plunger travel is normally of fixed length and is determined by the mechanical system which provides for the vertical movement of the plunger.
Such an arrangement suffers from disadvantages. Depending on the hardness of the crust and the level of the free surface of the electrolyte, which may vary by several centimeters, there is a double danger; if the duration of the downward movement is too short and/or the travel of the plunger is insufficient, the crust is not broken and the alumina cannot pass into the electrolyte. If the duration of the downward movement is too long and/or the travel of the plunger is excessive, a part of the plunger will be immersed in the molten electrolyte for a certain period of time.
This results in a crust being formed on the surface of the plunger, and that crust may increase in size in the following operations and gives rise to operational problems and causes progressive wear of the plunger.
Another problem which arises is that of local cooling of the electrolyte at the supply point. It frequently happens that when one of the anodes which is adjacent the supply orifice has just been changed and has not reached its condition of thermal equilibrium, the electrolyte solidifies completely at the supply point. The alumina which is poured out is not introduced into the bath and it further accentuates local cooling of the electrolyte.
In such a case, it would be desirable to stop the supply of alumina at that point and possibly to intensify the supply to the other points if any. However, that is generally not possible due to the lack of any system for automatically detecting this defect.
An object of the present invention is to make it possible on the one hand automatically to adjust the travel of the plunger to the level of the surface of the molten electrolyte and on the other hand to detect local solidification of the electrolyte at the respective supply point in question.